This invention relates to streams of particles and to methods and apparatus for deflecting such particle beams. More particularly, the invention is directed to a particle injection technique for use in material deposition devices such as plasma and flame spray guns.
Various techniques and devices for spray deposition of ceramic and metal compositions are known. Radio frequency and direct current plasmas have been used to deposit coatings on substrates as well as to fabricate articles of specific shapes and dimensions. In general, a stream of particulate material, such as metal or ceramic powder, is directed into the hot zone of a flame or a plasma where the particles are melted. The molten droplets are then directed to a target such as a mold or a substrate where they coalesce and form a solid article or coating.
One technique for spray-forming employs an RF plasma gun to generate a high temperature gaseous plasma. For example, U.S. Pat. No. 4,805,833, the disclosure of which is incorporated herein by reference, describes an RF plasma apparatus including an RF plasma gun. The plasma is produced by induced RF energy which causes gases flowing in the interior of the gun to form a plasma plume or jet which is directed to an adjacent substrate. In RF plasma deposition, powder particles of a feed material, entrained in a carrier gas, are introduced into the plasma. The heat of the plasma is sufficiently high to cause melting of the particles as they move through the plasma and deposition of liquid droplets onto the surface of the substrate
Particles injected into the center of the plasma drastically cool the small central plasma volume to which they are confined. This is shown theoretically in FIGS. 1 and 2. FIG. 1 shows the plasma temperature along the centerline as a function of axial distance at particle loadings of 0, 10, and 20 grams per minute. As the particle mass flow rate increases, cooling is increased. This cooling of the central portion of the plasma has an adverse effect on particle heating, thus limiting the maximum particle spray rate and also the melting temperatures of alloys which can be successfully sprayed. FIG. 2 shows the particle temperature as a function of loading rate. Plasma temperature and particle temperature are particularly important for spraying high temperature alloys which contain columbium,, molybdenum, or tungsten.